Alkylation of benzene with olefin of 8 to 16 carbon atoms produces alkylbenzenes that may find various commercial uses, e.g., alkylbenzenes can be sulfonated to produce detergents. As used herein, alkylbenzenes refers to phenylalkanes wherein the alkane group has between about 8 and 16 carbon atoms. Alkylbenzenes are produced as a commodity product in large-scale facilities, e.g. often in amounts of 50,000 to 200,000 metric tonnes per year per plant. In the alkylation process, benzene is reacted with an olefin the desired length to produce the sought alkylbenzene. The alkylation conditions comprise the presence of homogeneous or heterogeneous alkylation catalyst such as aluminum chloride, hydrogen fluoride, silica alumina or zeolitic catalysts and elevated temperature.
The alkylbenzene must meet stringent product specifications to be commercially acceptable. For instance, alkylbenzenes, to be desirable for making sulfonated surfactants, must be capable of providing a sulfonated product of suitable clarity, biodegradability and efficacy. The benzene content of the product should be relatively free from benzenes, e.g., less than about 1 part per million by weight (ppmw), and often less than about 0.5 ppmw. Also, desirable alkylbenzene products are relatively free, e.g., less than about 50, preferably less than about 5, ppmw, from byproducts such as dialkylbenzenes, oligomers of olefins, and the like (herein referred to as “heavies”). Additional considerations for commercial alkylbenzene products include the 2-phenyl content and linearity of the alkyl substituent. With respect to efficacy, alkylbenzenes having higher 2-phenyl contents are desired as they tend, when sulfonated, to provide surfactants having better detergency but less solubility if the 2-phenyl content becomes too high. Thus alkylbenzenes having a 2-phenyl isomer content in the range from about 25 to about 40 percent are particularly desired.
The catalysts are not selective and other reactions of olefins can occur to produce heavies, i.e., dimers and dialkylaryl compounds. Also, skeletal isomerization of the olefin can occur, resulting in a loss of selectivity to the sought alkylbenzene. The formation of dialkylaryl compounds is particularly problematic as the reaction approaches complete conversion of the olefin and the concentration of the alkylbenzene has thus increased thereby increasing the likelihood that an olefin molecule will react with an alkylbenzene molecule rather than benzene. Accordingly, typical processes use a large excess of benzene to reduce the molar ratio of the sought alkylbenzene to the olefin in the reactor. For homogeneous hydrogen fluoride catalyzed processes, the benzene to olefin ratio is generally in the range of 6:1 to 8:1. Solid catalysts are prone to generate more heavies. Hence, for these solid catalysts the mole ratio of benzene to olefin is typically greater than 15:1. For making alkylbenzenes with reduced skeletal isomerization, the benzene to olefin ratio is often in excess of 20:1 and sometimes as much as 30:1.
As the ratio of benzene to olefin increases, additional process costs are also incurred in the recovery and recycling of the unreacted benzene in the alkylation product. The refining system for alkylbenzene production is summarized in Pujado, Linear Alkylbenzene (LAB) Manufacture, Handbook of Petroleum Refining Processes, Second Edition, pp 1.53 to 1.66 (1996), especially pages 1.56 to 1.60. Especially for large-scale, commercial alkylation processes such as are used for the production of linear alkylbenzenes, capital and operating costs can be very important, and the addition of additional distillation steps can thus be undesirable. Of the distillations effected in the alkylbenzene refining system, the benzene distillation requires the highest heat duty.
Transalkylation of heavies from the alkylation of benzenes using solid catalysts has been proposed. However, the transalkylation requires benzene and, for purposes of regeneration, benzene is the preferred regenerant. Typically better catalyst performance is obtained with higher ratios of benzene to dialkylbenzene in the heavies being transalkylated. Often the mole ratios of benzene to dialkylbenzene are greater than 20:1. Accordingly, a transalkylation unit operation will be a significant consumer of benzene. Hence, the benzene distillation would have to produce incremental benzene for the transalkylation. This incremental production requires both a distillation assembly of sufficient size and additional reboiler heat. Transalkylation is not practiced in commercial complexes to make detergent range alkylbenzenes.
A desire exists for an integrated alkylbenzene production facility that can employ a transalkylation unit operation in an economically viable manner, including with acceptable catalyst stability, to enhance production of alkylbenzenes.